The present invention relates to brush gear for an electric motor and in particular to an arrangement for preventing shorting between brushleaves of the motor and commutators when the brushes have worn.
Brush gear in small electric motors, particular fractional horsepower permanent magnet direct current (PMDC) motors, typically comprises a carbon brush which is carried on an end of a resilient, electrically conducting arm--the brushleaf. The brushleaf is arranged to bias the brush against a commutator on the motor shaft. Typically the brush gear is mounted in an end cap which carries a bearing for the motor shaft and forms a structural part of the motor assembly.
As a brush wears down the supporting brushleaf moves closer to the commutator. There is a danger that the brushleaf may snag on the commutator, in particular that it may catch in between segments of the commutator, and be dragged around with the rotating commutator. This may result in the brush or brushleaf making electrical contact with the other brush or brushleaf, directly or via a common commutator segment. This shorting between the brushleaves can be very dangerous, leading to the risk of fire in some PMDC motor applications.
In GB patent specification No. 2201299A an arrangement is provided in which stops are used to prevent the brush leaf approaching too closely to the commutator. The stops are formed integrally with the plastic brush holding assembly. In practice, heat generated during use of the motor can lead to instability of the plastics material of the stop parts. In some cases to overcome this problem metallic stops have been used to replace the plastic stops but this requires some manner of a secure fixing of the motor parts in position and a separate step in production or the assembly of the motor parts.